In the field of wireless transmission, types of signal transmission are primarily classified into balanced transmission and unbalanced transmission. Each type of signal transmission, however, has its own advantages and disadvantages. To solve the above problems, two types of signal transmission are further integrated by wireless communication industries to form a device having functions of balanced transmission and unbalanced transmission at the same time, such that respective disadvantages are remedied. This device is mostly made up of ¼ wavelength transmission lines.
For instance, U.S. Pat. No. 6,441,696 B1 proposed a balun circuit comprising a λ/2 waveguide, a first λ/4 waveguide and a biasing means. A first end of the λ/2 waveguide is connected to a first port of the balun circuit, and a second end of the λ/2 waveguide is then connected to a second port of the balun circuit. A first end of the first λ/4 waveguide is connected to the second end of the λ/2 waveguide, while a second end of the first λ/4 waveguide is connected to a third port of the balun circuit. The biasing means is used for biasing components in a load connected between the first and second ports of the balun circuit. The biasing means comprises a second λ/4 waveguide, a first end of the second λ/4 waveguide being connected to the first end of the λ/2 waveguide, a second end of the second λ/4 waveguide being connected to a voltage source and to one electrode of a second capacitor, the other electrode of the second capacitor being connected to ground.
In the above prior art, the balun circuit is made up of ½ wavelength and ¼ wavelength waveguides. When it is necessary for the circuit to be provided in the manner of multi-balanced-unbalanced conversion, the overall area of circuit is grown by multiple times, and further, the available area is then limited. In addition, the required area of circuit is increased, and length of transmission line is also increased correspondingly. In this connection, higher loss of transmission is resulted, and overall characteristics of the converter are then reduced significantly.